Standard Test Method for Rapid Determination of Carbonate Content of Soils 1

Transcription

1 Designation: D Standard Test Method for Rapid Determination of Carbonate Content of Soils 1 This standard is issued under the fixed designation D 4373; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) indicates an editorial change since the last revision or reapproval. 1. Scope * 1.1 This test method covers the determination of carbonate content of soils and soft rock which can be readily broken down by mechanical effort. It is a gasometric method that utilizes a simple portable apparatus. Results should be clearly stated as the calcite equivalent in percent because different carbonate species cover a wide range of percent calcite equivalent as shown below for a number of carbonates: Species Cation Calcite Equivalent, % Magnesite Mg Dolomite Ca, Mg Calcite Ca Aragonite Ca Rhodocrosite Mn 87.1 Siderite Fe 86.4 Smithsonite Zn 79.8 Witherite Ba 50.7 Cerrusite Pb This test method is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.13 on Marine Geotechnics. Current edition approved July 10, Published November Originally published as D Last previous edition D For example, a 100 % dolomite would be expected to yield % calcite equivalent while 100 % siderite would yield only 86.4 % calcite equivalent. Calcite and aragonite reactions will typically complete within about 10 min. This method does not distinguish between the carbonate species and such determination must be made using quantitative chemical analysis methods such as atomic absorption. 1.2 The values stated in SI units are to be regarded as the standard. 1.3 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D The method used to specify how data are collected, calculated, or recorded in this standard is not directly related to the accuracy to which the data can be applied in design or other uses, or both. How one applies the results obtained using this standard is beyond its scope. 1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precaution statements, see Section Referenced Documents 2.1 ASTM Standards: C 25 Methods for Chemical Analysis of Limestone, Quicklime and Hydrated Lime 2 D 653 Terminology Relating to Soil, Rock, and Contained Fluids 3 D 3740 Practice for Minimum Requirements for Agencies Engaged in the Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction 3 D 4753 Specification for Evaluating, Selecting and Specifying Balances and Scales for Use in Soil, Rock and Construction Materials Testing 3 D 6026 Practice for Using Significant Digits in Calculating and Reporting Geotechnical Test Data 4 E 145 Specification for Gravity-Convection and Forced- Ventilation Ovens 5 3. Terminology 3.1 For definitions of terms used in this standard see Terminology D Summary of Test Method 4.1 The carbonate content (calcite equivalent) of soil is determined by treating a 1-g dried soil specimen with hydrochloric acid (HCl) in an enclosed reaction cylinder (reactor). Carbon dioxide (CO 2 ) gas is evolved during the reaction between the acid and carbonate fraction of the specimen. The resulting pressure generated in the closed reactor is proportional (see Fig. 1) to the calcite equivalent of the specimen. This pressure is measured with a suitable pressure gauge, or equivalent pressure-measuring device, that is pre-calibrated with reagent grade calcium carbonate. 5. Significance and Use 5.1 This test method is used to determine the presence and quantity of carbonate in a soil specimen in terms of the calcite equivalent. The method is generally intended for use as an 2 Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol Annual Book of ASTM Standards, Vol *A Summary of Changes section appears at the end of this standard. Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA , United States. 1

2 FIG. 1 Typical Calibration Curve for 0.374L Test Cell and 70 kpa (10 psi) Pressure Gage index of approximate carbonate content to assist with characterizing marine soils. Other test methods exist (such as Method C 25) to evaluate calcium carbonate equivalency for purposes of characterizing use of calcareous materials as soil modifiers or agricultural lining materials Calcium carbonates (CaCO 3 ) are known cementing agents, are water soluble at ph < 7, and are soft on the Mohs scale compared to other soil minerals. 5.2 This test method has limitations as follows: If low carbonate contents (calcite equivalents) are measured, the user does not know whether the soil is low in carbonate content or contains cerrusite, witherite, etc., which are carbonate species whose reactions with hydrochloric acid are either very slow or limited Testing times may be extensive (longer than 1 h) for some carbonate species (such as dolomite) if calcite equivalents within about 1 % are required The effects of specimen grain size, duration of testing, ph and specimen mass are discussed in the literature. 6 NOTE 1 The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D 3740 are generally considered capable of competent and objective testing/sampling/inspection, etc. Users of this standard are cautioned that compliance with Practice D 3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D 3740 provides a means of evaluating some of those factors. 6. Apparatus 6.1 Rapid Carbonate Analyzer A schematic drawing of 6 Demars, K.R., Chaney, R.C., Richter, J.A., The Rapid Carbonate Analyzer, Geotechnical Testing Journal, ASTM, Vol. 6, No. 1, March 1981, pp the rapid carbonate analyzer is shown in Fig. 2. The basic components of this apparatus include: Reaction Cylinder (Reactor), with threaded cap and O-ring seal to enclose the cylinder. A clear plastic cylinder allows viewing of effervescent reaction Pressure Gauge (Bourdon Tube-type or Electronic Pressure Transducer), 70 kpa (10 psi), with an accuracy of 0.25 %, and a readability of 0.5 kpa (0.1 psi) Acid Container, of clear plastic with a bail handle to hold 20 ml of acid. Optionally, in lieu of the acid container, use a soil container of clear plastic to hold 1gofsoil, with approximate dimensions of 25 mm diameter by 6 mm high with a nominal 4 mm notch down the side of the soil container. Acid added to dry soil often causes considerable splashing and use of soil container reduces splashing before reactor is sealed Pressure Relief Valve, for safe release of CO 2 gas pressure. 6.2 Balance The balance must meet the requirements of Specification D 4753 and this section. A Class GP1 balance with a minimum capacity of 100 g and a readability of 0.01 g is required to measure the soil mass. 6.3 Drying Oven, thermostatically-controlled, preferably of the forced-draft type, meeting the requirements of Specification E 145 and capable of maintaining a uniform temperature of C throughout the drying chamber. 7. Reagents 7.1 Purity of Reagents Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, 2

3 where such specifications are available. 7 Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination. 7.2 Calcium Carbonate (CaCO 3 ). 7.3 Hydrochloric Acid (HCl), (in about 1 N solution) Prepare 1 L of about 1 N solution by placing 80 ml of concentrated, reagent grade HCl in about 800 ml of commercial grade distilled water in a 1L volumetric flask Dilute to the mark with commercial grade distilled water. Store in polyethylene bottle. Faster reaction times may achieved by increasing the concentration to higher normal solutions (up to about 3 N solution). Hydrochloric acid is also commercially available in a1nsolution. 8. Safety Precautions 8.1 Use care in handling the hydrochloric acid so that no acid is spilled on either skin or clothing. If acid contacts the skin or eyes, immediately flush with large quantities of water. Process concentrated hydrochloric acid beneath a laboratory hood or in a well-ventilated area to reduce the inhalation of fumes. 7 Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD. 8.2 The pressure relief valve of the carbonate analyzer should be opened following each test to dissipate gas pressure so that the cap can be safely removed. 9. Test Specimens 9.1 Select 5 to 10-g specimens from a core or surface grab sample. Oven dry at C for a period of 12 to 24 h. Pulverize the entire sample with a mortar and pestle (or hammer) until all of the particles pass a No. 40 (0.425-mm) or finer sieve. Smaller particles react faster than larger particles when treated with acid. 10. Calibration 10.1 Calibration is accomplished by using reagent grade CaCO 3 to obtain the relationship between the mass of CaCO 3 and the pressure generated in the constant volume reactor. Each carbonate analyzer and pressure gage is individually calibrated. Prepare five sets of duplicate specimens with the following masses of CaCO 3 : Set 1 Two specimens at g, that is, analogous to 20 % calcium carbonate for a test specimen of 1 g Set 2 Two specimens at g Set 3 Two specimens at g Set 4 Two specimens at g Set 5 Two specimens at g Place the specimen into the reactor (using soil container if available). Add ml of acid solution into the 3

4 reactor (using acid container if available). Seal the reactor with top cap and close the pressure relief valve. Tilt the reactor to initiate reaction between the acid and specimen. Mix the contents by swirling. If available, place the reactor on a mechanical shaker for the duration of the test It is very rare that even calcite powder will completely react in less than 10 minutes. Therefore, it is essential to monitor the pressure to verify that the reaction is complete (pressure stabilized) and to confirm that the reactor is properly sealed (pressure does not decrease). Record the pressure reading to the nearest 0.5 kpa (0.1 psi) or better Open the pressure release valve and remove the reactor top cap. Dispose of the contents and rinse the reactor with distilled water Repeat 10.2 through 10.4 for remaining calibration specimens Data reduction may use any one or all of the following: Make a graph of percent calcite equivalent (using mass of calcite added) versus pressure in appropriate units for the gauge used Calibration data may be transferred to the face of the pressure gauge for direct reading of percent calcite equivalent as shown in Fig Use a least squares regression fit of the calibration data to arrive at an equation for calculating the percent calcite equivalent from the pressure data. 11. Procedure 11.1 Select a g specimen from the pulverized soil. Follow the procedures described in to obtain a direct measurement of the calcite equivalent. If there is no pressure response, but a minor effervescent reaction is observed, repeat the experiment with 2, 5, or 10 g of dry solids in the reactor and divide this carbonate reading by 2, 5, or 10, respectively to obtain the true calcite equivalent After a 10 min reaction time, read the pressure value to obtain the carbonate content. If one wishes to evaluate whether other carbonate species are present, take additional readings as shown in Continue reading pressure until the reaction is complete using the following criteria: (a) The change in calcite equivalent is less than 0.3 % over a ten-min time period for testing time up to 120 min; and (b) The change in calcite equivalent is less than 0.3 % over a 30-min time period for testing time greater than 120 min. 12. Report 12.1 Report all results to the nearest 1 % List sample source, project name and location (if applicable). 13. Precision and Bias 13.1 Precision Due to the nature of the soil and rock materials tested by this method it is either not feasible or too costly at this time to produce multiple specimens that have uniform physical properties. Any variation observed in the data is just as likely to be due to specimen variation as to operator or laboratory testing variation. Subcommittee D18.13 welcomes proposals that would allow for development of a valid precision statement Bias There is no accepted reference value for this test method, therefore, bias cannot be determined Some information on performance statistics of this test is provided in the literature Keywords 14.1 calcareous soils; calcite equivalent; carbonate content; Karbanat Bombe; marine soils 8 Dunn, D. A., Revised Techniques for Quantitative Calcium Carbonate Analysis Using the Karbanat-Bombe, and Comparisons to Other Quantitative Carbonate Analysis Methods, Journal of Sedimentary Petrology, Vol 50, 1980, pp SUMMARY OF CHANGES In accordance with ASTM Committee D18 policy, this section identifies the location of changes to this standard since the last edition (1996) that may impact the use of this standard. (1) Significant revisions to the standard were incorporated throughout most of the standard, including a name change (removal of word calcium ). (2) Scope Rewritten to explain that measurement of the carbonate content is actually the calcite equivalent and that the method also applies to soft rock. Added statements on use of Practice D (3) Addition of Referenced Documents and Terminology sections. (4) Summary of Test Method References to calcium carbonate revised to calcite equivalents and revisions for consistency in nomenclature for the reactor vessel. (5) Significance and Use Added discussion on limitations and reference to D (6) Apparatus Added oven specification, revised pressure gage and balance specifications, added optional soil container inside reactor. (7) Reagents Eliminated use of demineralized water. (8) Test Specimens Reduced amount of material to be prepared for the test. Specified finer sieve size for maximum particle size to speed up reaction times. (9) Calibration Added tolerances for masses of reagent and acid to be used. (10) Procedure Added options to increase specimen mass to obtain higher resolution for low carbonate content materials. (11) Precision and Bias Revised to conform with D18 policy. (12) Keywords Added calcite equivalent. (13) Summary of Changes Added. 4

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